The present invention relates to a direct-drive type electro-hydraulic servo valve of the type in which a spool is directly driven by a moving coil mounted on the spool and a stationary permanent magnet on the valve body.
Referring first to FIG. 1, a prior art direct-drive type electro-hydraulic servo valve will be described.
A sleeve 2 is fitted into a valve body 1 and a spool 3 is slidably fitted into the sleeve 2. A bobbin 4 upon which a coil 5 is mounted is securely attached to one end of the spool 3. A permanent magnet 6 is mounted on the valve body 1 so that a magnetic circuit is established between the coil 5 and the permanent magnet 6. When the coil 5 is energized, the spool 3 is caused to slide so that a desired intercommunication among oil passages 7, 8 and 9 within the valve body 1 can be established.
A displacement sensor 10 for sensing or detecting the position of the spool 3 is disposed at the other end of the spool 3 so as to determine the position of the spool 3 with respect to the sleeve 2. The output signal from the displacement sensor 10 is negatively fed back to the input of a power amplifier and is compared with a set point (not shown), thereby providing a feedback control system for stabilizing the spool 3.
Next the displacement of the spool is investigated. When the coil 5 is energized, the magnetic field is generated which interacts with the magnetic field of the permanent magnet 6. As a result, in response to the amperage and direction of the current, the spool 3 is displaced. Because of the above-described negative feedback from the displacement sensor 10, the spool 3 is stopped at a predetermined position and a liquid in quantity in proportion to the set point (not shown) can be supplied to a desired place.
However, when the spool 3 is driven or displaced in the manner described above, the spool 3 is oscillated as shown in FIG. 2a. Since the spool 3 is immersed entirely into a working oil filled within the sleeve 2, no damping action or brake is exerted to the spool 3 which is being displaced. Oscillation or vibration of the spool 3 causes oscillation or vibration of an actuator which is driven by the servo valve. Thus there exists a serious control problem.
There has been devised and demonstrated a method for decreasing response of a servo valve in order to prevent oscillation or vibration of the spool 3 (See FIG. 2b), but the high responsiveness of direct-drive type electro-hydraulic servo valves is thereby degraded.
Therefore a velocity sensor 11 is mounted on the spool 3 at the side of the bobbin 4 so as to sense or detect the velocity of the spool 3. The output of the velocity sensor 11 is negatively fed back to the power amplifier so that the spool 3 is damped.
Referring next to FIG. 3, the damping control will be described in detail.
R is a set point (opening or closing degree instruction) for the displacement of the spool 3 and is derived as an instruction from a servo-valve control system. K.sub.A is an electrical gain which is so controlled as to determine the response of the servo valve K.sub.X is an electrical gain of the displacement sensor 10 and remains unchanged once set. K.sub.B is a coefficient of a counter electromotive force produced when the coil 5 moves within the magnetic field of the permanent magnet 6. K.sub.F is a coefficient of a force exerted on the coil 5 when a current i is produced. K.sub.Q is a coefficient of the output flow rate Q which is dependent upon specfications of the servo valve. T is a time constant of the coil 5; m, the mass of the spool 3; b, a coefficient of the viscous damping exerted on the spool 3; F, the driving force for displacing the spool 3; v, the velocity of the spool 3; and s, a Laplace operator.
The damping force exerted on the spool 3 is dependent upon the viscous damping coefficient b and the coefficient of the counter electromotive force K.sub.B ; but such damping force is in general insufficient. Therefore in response to the velocity v detected by the velocity sensor 11, the damping force is multiplied with a suitable gain K.sub.V and negatively fed back.
Assume that the response of the coil 5 is fast enough (that is, the time constant T is almost zero), the loop gain from a signal V to a signal F becomes the product K.sub.V. K.sub.F. It is apparent therefore that it has the same dimension as the viscous damping coefficient b. That is, by effectng the negative feedback of v and by controlling suitably the value of K.sub.V the damping force can be applied to the spool 3. In practical usage, the effectiveness of the velocity feedback is influenced by the time constant T of the coil, but only slightly.
However, in the servo valve of the type described above, the velocity sensor 11 which is needed to control the damping is incorporated in a very limited space in the servo valve which is a precision device. As a result, the assembly is difficult, the cost is increased and the number of component parts of the velocity sensor is also increased. Therefore reliability is degraded. In addition, there arises a problem that if the velocity sensor is damaged, the whole servo valve must be replaced with a new one.
The present invention was made to overcome the above and other problems encountered in the prior art and has as its object to provide a direct-drive type electro-hydraulic servo valve in which damping can be exerted on a servo valve without providing a velocity sensor in the main body of the servo valve.